1. Field of the Invention
The present invention relates to a sequential comparison type AD converter and microcomputer.
2. Description of the Related Art
A sequential comparison type AD converter has series resistors and is designed to sequentially compare a reference value and analog value generated at each connection portion of these series resistors, by a method of ½ approximation, to convert the analog value into an m-bit digital value. The series resistors consist of 2m resistors which are connected in series depending on the conversion accuracy (e.g., m bits) of the sequential comparison type AD converter. That is, together with improvement of the conversion accuracy (resolution) of the sequential comparison type AD converter, the number of series resistors increases dramatically by the power. For example, if the conversion accuracy of the sequential comparison type AD converter is improved from 8 to 10 bits, then the number of series resistors increases from 256 to 1024. Consequently, in the sequential comparison type AD converter, because the number of series resistors is large, this results in the problem that pattern area of the series resistors and the production cost increase in addition, there is the problem that in the microcomputer in which the sequential comparison type AD converter is incorporated, the chip area increases.
The applicant of the present invention has filed the application of a sequential comparison type AD converter aiming to solve the above problems (see e.g., Japanese Patent Application Laid-open Publication No. 2001-53612). This sequential comparison type AD converter improves conversion accuracy by connecting in parallel a plurality of capacitive elements between a ground and an input of a comparator where a reference value is input, and using as the reference value an average value of the charging voltages of the plurality of capacitive elements.
However, in the sequential comparison type AD converter of Japanese Patent Application Laid-open Publication NO. 2001-53612, although the increase in the number of series resistors arising from improvement of conversion accuracy can be prevented, it is necessary to suitably control the charging timing of the plurality of capacitive elements, thus resulting in the problems that the number of control elements for controlling the timing increases and that required control becomes complex.